In a jar of oily liquid float millions of tiny magnetic grains: each about the size of a bacterium. Switch on a magnet, and the grains, like obedient little soldiers, line up into long threads. But why do they form threads rather than clumps? It turns out it's not just about magnetism.
Each particle, as it moves through the liquid, drags a flow along with it, like an oar leaving swirls in water. These flows, created by neighbors, push particles toward each other and align them in a line. Without such flows, particles would clump into shapeless bunches. Scientists simulated this process on a computer and saw that hydrodynamic interactions, as these mutual pushes through the fluid are called, play a key role.
Understanding exactly how threads form opens the way to creating materials with a desired structure. For example, you could make gaskets that tighten only where needed, or protective shields that become stronger on impact. Now engineers can control not just how much the fluid thickens, but also the shape that particles take inside it.
Magnetorheological fluids are used in knee prosthetics: a microprocessor changes the fluid's viscosity depending on the phase of the step, allowing a person with a prosthetic to climb stairs almost naturally.